The Fabry-Perot (FP) interferometer has played a seminal role in the development of a number of scientific fields including atomic and molecular spectroscopy, material science, astronomy, lasers and optical communications. Fabry-Perot etalons are optical filters based on the FP interferometer. These filter are capable of providing extremely high resolution approaching tens of megaHertz at optical frequencies, very high light throughput and can comprise active optical components capable of fine wavelength tuning.
The FP etalon optical filter operates by multiple-beam interference of light reflected and transmitted by a pair of parallel, flat optical reflectors. In its most basic configuration, a FP etalon filter comprises an optical resonance cavity formed between two partially reflective, low loss reflectors. Typically, the resonance cavity is an air gap or a dielectric material having an optical path length equal to an integer multiple of one half the wavelength of light to be transmitted by the filter and the reflectors comprise dielectric stacks of alternating high and low refractive index layers. Such FP etalon designs have been used in both pulsed and continuous wave optical filtering applications and have been demonstrated to effectively filter light over the ultraviolet, visible and infrared spectral regions.
Light propagating through the FP etalon is partially transmitted and reflected upon every interaction with each reflector. The parallel optical configuration of the FP etalon provides for multiple reflections, which results in constructive and/or destructive interference depending on the wavelength of light propagating through the cavity. As a result of optical interference of reflected and transmitted beams, only certain frequencies of light are transmitted by the FP etalon filter. The transmitted frequencies correspond to the resonance frequencies of the etalon filter. Therefore, etalon transmission spectra typically comprise a plurality of transmission bands that are evenly separated from one another by the free spectral range of the etalon filter. The resonance frequency, free spectral range, light throughput and band width of transmission bands depend on several parameters including: (1) the optical path length of the resonance cavity, (2) the index of refraction and index dispersion of the cavity (3) reflectance of the reflectors and (4) extent of parallelism of the pair of reflectors. FP etalon filters of this type are extensively described in by Moore et al. in “Building Scientific Apparatus”, Addison—Wesley Publishing Co, 1989, pgs 242-251 and Hect in “Optics, 2nd Edition”, Addison—Wesley Publishing Co, 1987, pgs. 368-372.
In long-haul telecommunication systems, optical signals are often generated from electronic signals, transported great distances along optical fiber networks and detected in a manner to regenerate the original electronic signal. While such telecommunication systems exploit the substantial efficiency gains of using optical methods for signal transmission, signal processing via standard electronic techniques remains a barrier to achieving the highest overall efficiency and accuracy of the telecommunication network. Specifically, the use of electronic components for signal processing often imposes substantial physical limitations on the speed that information can be transmitted and utilized. Accordingly, to fully realize the efficiency and accuracy possible in a purely optical telecommunication system, a need currently exists for optical communication technology capable of direct optical processing of telecommunication signals. Examples of such optical signal generation and processing applications include wavelength stabilization techniques, dispersion compensation methods and wavelength multiplexing techniques and are described in detail in U.S. Pat. Nos. 5,798,859, 6,208,444, 6,169,626 and 5,999,320.
Improvements in purely optical signal processing have focused on development of optical devices capable of all aspects of signal generation, processing and detection. These efforts include research into new optical devices that perform a variety of signal transmission and processing functions including signal amplification, beam splitting, signal coupling, optical filtering, multiplexing, demultiplexing optical switching and dispersion correction. High throughput optical filters with selectable transmission frequencies and bandwidth are essential components of a wide variety of such optical devices. FP etalon filters capable of providing these functions are of great importance to the development of highly efficient and accurate optical telecommunication systems.
Wavelength division multiplexing is used to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber. In wavelength division multiplexing, a plurality of optical signals of different wavelength are multiplexed by coupling each signal to a common transmission line. The multiplexed transmission signal is then propagated over a single optical medium to a variety of receivers. When received, the (multiplexed transmission signal is demultiplexed into separate channels corresponding to individual wavelengths and detected by a receiver. Typically, signal demultiplexing is achieved by a variety of wavelength selective optical filtering devices including optical interference filters, cutoff filters, prisms and diffraction gratings. Although wavelength division multiplexing provides a simple, effective and low cost way of increasing transmission capacity, the feasibility of this technology is dependent on the development of high resolution, high throughput filters and transmitting lasers with highly stable and accurate frequencies.
Adoption of universal standard transmission channels for fiber optic transmission promotes efficient application of wavelength division multiplexing. The International Telecommunication Union (ITU) has adopted a standard channel definition providing a 45 channel system over a wavelength range of 1533 nm to 1565 nm with a uniform channel spacing of 100 GHz (approximately 0.8 nm). High resolution, high throughput optical filters capable of acting as a reference for the ITU standards are needed in the art.
The high throughput nature of FP etalon filters makes their use as optical frequency discriminators in devices that generate, detect or process optical signals especially attractive. While the periodic nature of the transmission spectra of FP etalon filters makes them ideally suited for wavelength discrimination in multiplex applications employing equally spaced transmission channels, their use is currently hampered by fundamental spectral limitations impeding accurate frequency matching of the transmission bands of a standard FP etalon filter with the transmission channels defined by the ITU frequency standard. These spectral limitations include the inability of FP etalon filters of the prior art to have the appropriate free spectral range and resonance frequency to overlap the transmission channels of the ITU frequency standard.
Both the free spectral range (Δν) of a FP optical filter and the resonance frequency (ν) depend on the optical path length (L) of the resonance cavity and the cumulative reflectance phase dispersion of the reflectors. As a result, most FP etalon designs currently available do not provide independent and selectable control of free spectral range and resonance frequency. Rather, etalons in the prior art only provide independent selection of either one of these two variables. Unfortunately, these designs do not provide FP etalon filters with transmission spectra that match the channel spacing and position of the universally adopted ITU frequency standard.
The ability to selectively and independently control the resonance frequency and free spectral range of an etalon optical filter would aid tremendously in realizing the full potential of wavelength division multiplexing in optical telecommunication systems. First, FP etalon filters with transmission spectra selected to match the adopted transmission channels of the ITU frequency standard would provide a universally applicable frequency reference for distributed feedback lasers, which comprise an important optical source for telecommunications signaling. Specifically, these filters would possess the high resolution needed for laser frequency monitoring and control at all transmission frequencies of the ITU transmission channels, within the narrow tolerances needed for efficient signal multiplexing, which can approach 10-20 ppm.
Second, FP etalon filters with independently adjustable resonance frequency and free spectral range would provide frequency discriminators ideally suited for signal demultiplexing applications. In particular, these filters would allow selectable, high throughput transmission of light corresponding to one or more transmission channels in the ITU frequency grid. Such etalon optical filters would provide accurate wavelength discrimination and signal processing with minimal loss of signal.
Finally, FP etalon filters with independently adjustable resonance frequency and free spectral range would provide important means for correcting chromatic dispersion inherent, to wavelength multiplex signals that propagate over long fiber distances. Chromatic dispersion is caused by the dependence of the refractive index of silica on wavelength and causes different parts of the signal spectrum to arrive at the distant end of the system at different times. FP etalon filters can be used to compensate for chromatic dispersion because the optical frequency of any portion of the signal contains the information of the delay that has occurred.
U.S. Pat. No. 5,212,584 discloses a tunable etalon filter for wavelength division multiplex optical communication systems. Specifically, the etalon design disclosed is reported to provide selectable control of resonance frequency by employing a temperature controlled resonance cavity comprising a spacer composed of a material with a relatively large rate of change of refractive index with temperature. While the resonance frequency is reported to vary systematically with angle of incidence and cavity temperature, the etalon described in U.S. Pat. No. 5,212,584 does not provide substantially independent selectable resonance frequency and free spectral range. Therefore, selection of the resonance frequency of the etalon fixes the free spectral range to a set value. Accordingly, U.S. Pat. No. 5,212, 584 does not disclose methods of frequency matching the transmission bands of an etalon optical filter with the plurality of transmission channels or emission lines of a given frequency standard, such as the ITU frequency grid.
U.S. Pat. No. 5,291,332 discloses FP etalon designs having selected reflectance phase dispersion characteristics, which are reported to match aperiodic atmospheric spectral lines. The FP etalon design described employs reflectors having rugate coatings selected to achieve a prescribed reflectance phase dispersion. Specifically, the phase and frequency of the sinusoidal index of refraction profile of the rugate coating is selected to achieve the desired FP etalon transmission characteristics. While U.S. Pat. No. 5,291,332 reports successful frequency matching of the etalon transmission spectra and aperiodic atmospheric spectral lines, the reference does not disclose methods of frequency matching periodic spectral lines. Particularly, the reference does not disclose or suggest devices or methods capable of independently adjusting etalon resonance frequency and free spectral range while preserving a substantially periodic transmission spectrum. Further, the reference does not disclose techniques for frequency matching etalon transmission spectra and the evenly spaced transmission lines of given frequency standard or the evenly spaced lines of an optical source. Finally, the etalon design described in U.S. Pat. No. 5,291,332 employs costly rugate reflectors that are difficult to manufacture.
U.S. Pat. No. 6,154,318 discloses dispersive multilayer mirror structures that reportedly provide selectable reflectance group delay. Specifically, the authors report the use of a multilayer sequence of thin dielectric films to provide selectable adjustment of the reflectance group delay by using multiple resonance trapping techniques. Although the mirror structures disclosed are reported to provide reflection at selected frequencies, U.S. Pat. No. 6,154,318 does not disclose optical methods for frequency filtering employing Fabry-Perot interferometry. Further, the methods disclosed are limited to the use of multilayer mirror structures for pulsed optical source applications. Finally, the methods and devices disclosed by the authors to achieve the desired resonance trapping properties are limited to “resonant substructures arranged around layers less than one quarter-wavelength optical thickness.”
It will be appreciated from the foregoing that a need exists for FP optical filters with independently selectable resonance frequency and free spectral range. The present invention provides high throughput FP etalon filters able to provide substantially independent selection of both resonance frequency and free spectral range. Further, the present invention provides FP etalon structures that are capable of frequency matching optical signals to the transmission channels of any selected frequency standard, particularly the International Telecommunications Union frequency standard.